Cordierite ceramic honeycomb articles have broad applications in automotive, environmental and chemical industries. Cordierite honeycomb articles are used as an exhaust gas purifying catalyst carrier, a filter, or a heat exchanger. In automotive applications, cordierite honeycomb articles have been used as a catalyst carrier for exhaust gas purification. The exhaust gases emitted by internal combustion system include hydrocarbons (HC), carbon monoxide (CO) and nitric oxides (NOx). Automotive, material, and environmental industries have worked intensively to reduce the amount of emission gases and to meet the tightened regulations every year.
To improve emission control efficiency of honeycomb articles such as monoliths, it is desirable to have a ceramic honeycomb carrier having certain properties. These properties include low thermal mass for fast light-off and better catalytic performance at lower temperature; low bulk density but with the same geometric surface area (GSA) for reducing heat capacity; low pressure loss for improved fuel consumption; good mechanical strength; and high thermal shock resistance.
To achieve the above-described desirable properties, thin-wall and ultra-thin-wall honeycomb articles have been designed and developed to meet the fast light-off and lower pressure loss requirements. The reduction of the honeycomb wall thickness, however, is difficult in the production technology because, in general, reduction of the honeycomb wall thickness accompanies a decline in the mechanical strength of the honeycomb article itself.
In manufacturing practice, the amount of the material, the particle packing and the extrusion pressure required for forming the skin of the honeycomb articles differ from those required for forming the webs of the honeycomb articles. Normally, in honeycomb articles, the thickness of the skin is greater than the thickness of the webs; therefore, the skin and webs are inevitably unbalanced. If the skin is made thicker, then cell deformation defects occurs. If, on the other hand, the skin is made thinner, then fissure (cracking) of the skin may occur. In cordierite thin-wall and ultra-thin-wall honeycomb articles, several defects have been observed in the outer wall including straight vertical fissures, vertical fissures jagged within a single cell, vertical fissures jagged within several cells, and horizontal ring-off cracks. Chips, bumps and groves are also resources of defects and cracks. The formation of ceramic honeycomb articles having good quality skin extrude to shape skin is desired.
Various methods have been proposed to solve the above-mentioned problems. Each of these approaches, however, have disadvantages including a mismatch in the coefficient of thermal expansion (CTE) of the reinforcing material and the coefficient of thermal expansion of the honeycomb structure, an overall decrease in strength, and a decrease in gas purification efficiency.
During typical production processes, a cordierite honeycomb substrate experiences a temperature difference in the furnace. When it is coated with catalyst, the honeycomb substrate is treated with a certain temperature gradient. When it is used as a catalyst carrier for an exhaust gas purifier, a temperature difference is generated in the catalyst carrier because of heat released by the oxidation reaction of the unburned hydrocarbon and carbon monoxide present in the exhaust gas. All of the above-mentioned proposed methods require that the honeycomb substrate have higher thermal shock resistance and a capability to reduce or disperse the thermal stress. The thermal shock resistance is related to several properties including, for example, thermal expansion coefficient, elastic modulus, strength, porosity, pore size distribution and pore shape. Increasing thermal shock resistance is important to minimize or prevent the risk of cracking and damage in catalyst carriers.
Cordierite is a desired material for the manufacture of honeycomb articles. Typically, the present invention is used in automotive exhaust gas treatment systems, such as ultra-thin-wall products, generally having a wall thickness of less than 4 mil. Other uses include high temperature articles, such as catalytic converters, NOx adsorber substrates, catalyst substrates, and diesel particulate filters because of its relatively low cost. Cordierite materials are typically manufactured by mixing a raw batch that includes talc, alumina, aluminum hydroxide, kaolin and silica. The batch is then blended with a binder (such as methylcellulose) and a lubricant to form a plastic mixture. This plastic mixture is then passed through a die, formed into a green body, and sintered. U.S. Pat. No. 6,864,198 (hereby incorporated by reference in its entirety) assigned to Corning Incorporated discloses one method of forming a cordierite honeycomb structure.
Accordingly, cordierite honeycomb articles structures having no defects such as fissures and/or cracks are much sought after. Methods of manufacturing such honeycomb articles are similarly sought.